Of all the weather phenomena seen in the hills, a cloud inversion is perhaps the most breathtaking. Your day may have started in the dense, dank clag of the valley, but time it right and things begin to look up as you climb, until suddenly you find yourself marooned on a peak rising into the sunshine like an island in a sea of cloud. Cloud inversions may be most common in autumn and winter, but they are still elusive, requiring quite specific conditions to form. Here's some advice from film maker Terry Abraham and MWIS meteorologist Geoff Monk on how to maximise your chance of finding one.
"Generally it is in the winter half of the year when inversions descend onto the British mountains so that higher tops are warmer than lower slopes above a haze or low cloud/fog layer" Geoff Monk, MWIS
In normal daytime conditions the sun warms the ground, and the ground warms the air. As you gain height, walking uphill for instance, the air temperature will tend to drop. But sometimes this standard temperature gradient can be reversed, with colder air low down in the valleys trapped beneath a capping layer of warmer air that sits on the hill tops. With luck the colder layer will be full of fog or cloud - the magic cloud inversion. If you're hoping to catch that special combination of low level cloud and high level sunshine, it pays to have a basic understanding of how it all works:
High pressure weather systems - known as anticyclones - bring long periods of stable weather, typically with little wind to break up any cloud. With the long nights and comparatively weak sunshine of late autumn and winter the ground cools, and so too the air at low levels. If the wind brings sufficient moisture off the sea into the weather system it can condense as cloud in the colder air layer. When this colder air is trapped low down by a cap of warmer air the cloud may linger for days. Depending on the height at which the inversion occurs, hilltops might poke free into the clear air above. If you're watching the weather charts look out for high pressure systems, which appear as a series of widely spaced concentric rings, or isobars, of 1000mbs and more.
Cold air sinks
On clear, calm nights colder air flows downhill to end up sitting in valley bottoms. As air cools it becomes saturated, the moisture condensing into cloud. Come morning there may be a cold winter fog down in the valley, but climb a few hundred metres and you could - with luck - find yourself above it.
However in practise forecasting all this isn't simple:
'Generally it is in the winter half of the year when inversions descend onto the Scottish mountains so that higher tops are warmer than lower slopes above a haze or low cloud/fog layer' says meteorologist Geoff Monk of the Mountain Weather Information Service (MWIS).
'This occurs generally, but not necessarily, in high pressure. However, we cannot say that the higher the pressure the more likely there will be inversion conditions. This might be truer over the Alps, but we are affected very much by the sea.'
'Most wintertime major builds of pressure occur in the wake of a cold northerly wind. In the cold air, the air will 'convect' over the sea. This means cumulus clouds rising vertically will be pushing the developing inversion (as pressure rises) back up, restricting the inversion height to above 1500m. So very often, even under high pressure, the inversion just doesn't descend onto the British summits - at least not for a few days, when after the cold air source has been cut-off, convection progressively becomes less vigorous, allowing the inversion to sink. In these common situations, forecasting on just which day higher summits will be above the inversion is not easy.'
"When I check the weather forecast and any relevant synoptic charts I will look to see if the wind speeds are relatively calm, if the night will be clear and how deep and sheltered are the valleys near my camp" Terry Abraham
It takes dedication to acquire a knack for predicting these things. Film maker Terry Abraham has made something of a speciality out of capturing spectacular cloud inversions, frequently filmed from high camps on mountain tops. His secret? Keeping a weather eye on the forecasts, and getting out when - and where - the going looks good:
'When I check the weather forecast and any relevant synoptic charts I will look to see if the wind speeds are relatively calm, if the night will be clear and how deep and sheltered are the valleys near my camp' he says.
'Always endeavour to go where the weather tells you to - and not plan to spend the night somewhere merely in the hope you'll catch a temperature inversion. Note the terrain, too. A prominent peak looming over sheltered valleys would be a safe bet.'
'Valleys with large bodies of water, such as those found in the Lake District, are ripe for such scenes too, due to cooling water.'
In planning a location Terry spends time thinking how the weather is likely to interact with the topography. With a northeasterly air flow, for instance, a hill like Ingleborough would be a good bet for an inversion, he says.
'If the winds are westerly it can be something of a contrast though. In the Peak District you'd think Kinder Scout would be good because of its altitude. But it's a big mass, so on its western side the pressure it would force into any incoming winds would be considerable - and in turn you'd likely get the cloud creep its way over the plateau and not see much at all. Instead head for somewhere like Grindslow Knoll, a little prominent hill in the lee of Kinder.'
Terry is probably best known for his films of the Lake District. Here's an example from his blog of the lengths he'll go to in order to catch his trademark summit cloud inversions:
'The previous few days had been generally calm, clear and cold. But one evening I noted a haze forming in the valleys around Keswick. It made for a fantastic sunset as it set the world alight in glorious shades of pink.'
'I knew after looking at the weather forecast that a temperature inversion was guaranteed. Winds and cold air were north easterlies and there happened to be a massive bank of cloud coming in from the Irish Sea with very little wind.'
'With this in mind I set off at 3am from the holiday cottage I was staying in with my family - catching sight of the odd late night reveller in the town centre as I made my way to the flanks of Latrigg Fell. By this point visibility was only 30 feet or so and got less as I gained altitude making my way for the summit of Skiddaw on New Years Eve.'
'It was freezing cold, the ground was like concrete and progress was slow in the dark. Moisture formed on my eyelashes, my vision became blurred and morale was a little low as I finally began the steep ascent of the 'tourist path' up to Little Man. At times, when I stopped for a quick breather, I did think "What the hell am I doing here at 4 in the morning?!!". Some will think it's crazy but it makes for a good adventure with a fantastic reward.'
'Because not so long up this arduous and boring route, I noted a twinkle or two up ahead in the murk. At first I figured it was my headtorch playing tricks with moisture on my eyes and carried on regardless, thinking it won't be stars in a clear night sky.'
'But after catching sight of a few more sparkles up ahead, I stopped dead in my tracks, turned off my torch and made a slow turn to look behind and then BOOM! I was punching the air with delight! A sheet of thick cloud as far as the eye could see on every point of the compass was just lapping below me.'
'It was mesmerising but I didn't stick around for long as by now my spirits had shot off into the stratosphere and I suddenly found the energy of ten men in my legs to sprint up to the summit of Little Man - whereupon my rucksack was thrown onto the ground, my cameras were set up and a gas stove was put on the go for a cuppa and breakfast.'
'I truly was in heaven that day and it's one that will linger long in my memory. I didn't want to go back down into the murk. It was quite simply breathtakingly beautiful up there and it wasn't until an hour before sunset that I reluctantly made my way back down to Keswick!'
'My wife knew where I had been but was still a little annoyed I got home quite late and then spent the rest of the evening and its consequent celebrations asleep on the sofa.'
- Terry Abraham is currently working on the film Life of a Mountain: Blencathra. Catch up with the latest from him on Facebook
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